Rapid warm-up of engine fluid, engine oil, and transmission fluid is important to fuel economy during a cold start (i.e., when the vehicle has not been running and the engine and transmission are relatively cold). Engine warm-up is especially challenging for diesel and hybrid applications, as less fuel is burned. Adequate passenger compartment heater performance is also necessary for passenger comfort. Powering the heater using a motor/generator on a hybrid vehicle can negatively impact fuel economy.